The present invention relates to a method for sterilizing liquids by brief heating and to apparatus for performing the method.
Increasing use has been made recently of sterilizing liquids for storage followed by packaging of such liquids in sterile packages under sterile conditions in order to permit prolonged storage of such liquids, more particularly liquid foodstuffs such as milk, cream and fruit juices. Different methods are used for sterilizing a liquid, for example the mechanical treatment of the liquid with repeated pressure variations, usually of the ultrasonic kind. The most common method of sterilizing liquid however is heating the same for a sufficiently long period of time in order to inactivate all microorganisms and spores which may be present in the liquid. All the previously-mentioned sterilizing methods must be adapted to the kind of treated liquid, for example it is often inappropriate to sterilize liquid foodstuffs by means of a chemical sterilizing medium because this may also render the liquid unsuitable as foodstuff while it may be advantageous for other liquids, for example effluents containing certain kinds of bacteria, to be sterilized by means of sterilizing media.
Although the method according to the present invention can be used for sterilizing most liquid products its most important field of use is undoubtedly the sterilization of milk and other liquid dairy products and fruit juices. Heat sterilization, for example of milk, causes this to assume an unpleasant cooked flavour and the milk assumes a slight brown discoloration which also substantially impairs the quality of the milk. One method of avoiding this flavour deterioration and discoloration of the milk is by heating it intensively for a short period of time, i.e. for approximately 2-5 seconds, to approximately 130.degree.-150.degree.C followed by instant cooling to a temperature below 80.degree.-90.degree.C. Only very small quantities of chemical reaction products which impart an unpleasant flavour to the milk can be formed in such short heating of the milk and this is also not subjected to any noticeable discoloration. Several methods all of which provide a satisfactory sterilization result without any evident impairment of quality have been developed for such short-period heating of the milk. However, these methods suffer from the disadvantage that they consume a large quantity of energy and are therefore expensive.
In the method according to the present invention the liquid is also treated so that it is heated for a short period to the sterilizing temperature and is then instantly cooled but by comparison with known methods the method according to the invention requires substantially less energy.